Hi
@Phi.
I am afraid there is no block/cap you can put on your phone to stop you from going over your monthly tariff. I understand Tesco, who use the O2 network, do offer this facility but not O2.
The only suggestion I can make is to keep a regular check in My O2 on your phone/text usage if you're not on unlimited calls and texts. Your data will automatically stop when your limit is reached.
The only other alternative is either to increase your monthly tariff or switch to a PAYG account.
Lover of all things Samsung. Currently using Samsung Galaxy S10 Plus 128gb